1. Field of the Invention
The present invention relates to a cathode ray tube which enhances a speed modulation effect.
2. Description of the Related Art
A color cathode ray tube, particularly a high brightness cathode ray tube such as a projection-type cathode ray tube forms images of high brightness and high definition on a phosphor screen by increasing electron beams (current) projected to a phosphor screen, by increasing an acceleration voltage applied to a final acceleration electrode (anode), and by elevating a potential of a focusing electrode.
Further, there has been known a method which changes a scanning speed of electron beams in response to a contrast level of images to display images having an excellent contrast (speed modulation method).
In this method, the scanning of electron beams is controlled such that when the electron beams perform horizontal scanning from a black level to a white level in response to a differential output of image signals, the scanning speed is temporarily accelerated and thereafter the scanning is temporarily stopped, while when the electron beams perform horizontal scanning from the white level to the black level in response to a differential output of image signals, the scanning is temporarily stopped and thereafter is temporarily accelerated.
A portion where the scanning speed is fast exhibits low electron beam density and hence, the portion is dark, while a portion where the scanning is stopped exhibits the high electron beam density and hence, the portion is bright. Accordingly, a region of black level is increased and, at the same time, a region of white level is narrowed so that the current density is increased where by the brightness is increased. Accordingly, the contrast is enhanced so that an image display of high quality is obtained.
An evacuated envelope of a cathode ray tube is constituted of a panel portion on which a phosphor screen is formed, a neck portion which houses an electron gun and a funnel portion which connects the panel portion and the neck portion.
FIG. 15 is a cross-sectional view of a neighborhood of a neck portion of a conventional cathode ray tube. An electron gun is housed in the neck portion 23. The electron gun is constituted of a cathode K, a first grid electrode (control electrode) 11, a second grid electrode (accelerating electrode) 12, a third grid electrode (front-stage anode electrode) 13, a fourth grid electrode (focus electrode) 14 and a fifth grid electrode (anode electrode) 15. A deflection yoke 6 is exteriorly mounted on a transitional region between the neck portion 23 and the funnel portion 22. Further, on an outside of the neck portion 23, a correction magnetic device 7 for convergence adjustment and color purity adjustment and a speed modulation coil 8 are exteriorly mounted.
Electron beams temporarily receive a positive deflection action (scanning direction) or a negative deflection action (direction opposite to scanning direction) in the horizontal scanning direction due to a magnetic field generated by the speed modulation coil 8.
An electric current which flows in the speed modulation coil8 has a high frequency and the fourth electrode 14 is constituted of nonmagnetic metal material such as stainless steel in the same manner as other electrodes and hence, when the magnetic field generated by the speed modulation coil 8 acts on the electrode 14, an eddy current is generated in the inside of the electrode 14.
The generation of a magnetic flux which acts in an inner space of the fourth electrode 14 is suppressed by this eddy current so that the speed modulation effect is reduced.
To make the speed modulation magnetic field effectively act on the electron beams, it has been known to divide the fourth electrode 14 into halves along an electron beam path. The divided halves of the fourth electrode 14 are electrically connected by a connection line.
Due to such a constitution, it is possible to perform the speed modulation by inserting the magnetic field of the speed modulation coil in the space of the fourth electrode 14 so that the highly efficient speed modulation can be realized.
Further, by elongating an interval in the tube axis direction of the two-split fourth electrode 14, the speed modulation magnetic field acts on the electron beams more effectively.
FIG. 16 is a side view of an electron gun adopting a speed modulation method. In the electron gun shown in FIG. 16, a portion of the fourth grid electrode 14 is inserted into the fifth grid electrode 15. In FIG. 16, parts which perform the same actions as the parts shown in FIG. 15 are indicated by the same numerals.
As publications which disclose the prior art related to this type of cathode ray tubes, for example, Japanese Laid-open Patent Publication 334824/1998, Japanese Laid-open Patent Publication 74465/1998 and Japanese Accepted Patent Publication 21216/1987 are named.
Further, a structure in which a coil-shaped portion is formed in a portion of a third grid electrode is disclosed in Japanese Laid-open Patent Publication 188067/2000.
In an electron gun which divides a focus electrode into halves in the tube axis direction, there exists a limit with respect to the expansion of a gap between the divided halves. When the gap between the divided halves of the electrode is excessively large, it is impossible to maintain the potential in the inside of the fourth electrode at an equal potential. That is, when the gap between the divided halves of the electrode is increased, the electron beams receive the influence of an electric field other than the electric field generated by electrodes of the electron gun or an external magnetic field. For example, the influence of electric fields from a charged with the front-stage anode electrode 13 by means of a connection line 181.
Since the front-stage anode 13 and the focus electrode 14 are respectively divided, an eddy current which is generated in the focus electrode 14 due to a magnetic field generated by the speed modulation coil 8 is reduced. Further, the magnetic field generated by the speed modulation coil 8 can easily enter the electron beam passing region so that a sufficient speed modulation effect can be obtained. Accordingly, a contrast of displayed images can be enhanced.
In FIG. 2, the front-stage anode electrode 13 has one gap and the focus electrode 14 has three gaps. However, the anode electrode 13 may have a plurality of gaps and the focus electrode 14 may have a single gap. In this embodiment, to make the speed modulation magnetic field permeate into the inside of the focus electrode 14 where the diameter of the electron beam becomes bold as much as possible, three gaps are formed in the inside of the focus electrode 14.
The third focus electrode 143 uses parts having the same shape as those of the second focus electrode 142.
In FIG. 2, A1 indicates a total length of the first front-stage anode 131, A2 indicates a total length of the second front-stage anode 132, B1 indicates a total length of the first focus electrode 141, B2 indicates a total length of the second focus electrode 142, B3 indicates a total length of the third focus electrode 143, B4 indicates a total length of the fourth focus electrode 144, C1 indicates the gap between the first front-stage anode 131 and the second front-stage anode 132, D1 indicates the gap between the first focus electrode 141 and the second focus electrode 142, D2 indicates the gap between the second focus electrode 142 and the third focus electrode 143, D3 indicates the gap between the third focus electrode 143 and the fourth focus electrode 144, E1 indicates an interval between the second front-stage anode 132 and the first focus electrode 141, xcfx861 indicates an inner diameter of the second front-stage anode electrode 132 and an inner diameter of the first focus electrode 141, and xcfx862 indicates an inner diameter of the large-diameter portion of the fourth focus electrode 144.
The length extending from a phosphor-screen-side end portion of the first front-stage anode 131 to a cathode-side end portion of the fourth focus electrode 144 (C1+A2+E1+B1+D1+B2+D2+B3+D3) is 20 mm. By setting the length extending from a phosphor-screen-side end portion of the first front-stage anode 131 to a cathode-side end portion of the fourth focus electrode 144 equal to the total length of the speed modulation coil 8, the magnetic field which is generated by the speed modulation coil 8 can be effectively utilized.
With the provision of cathode ray tube using the electron gun 5 having the constitution of this embodiment, the magnetic field generated by the speed modulation coil 8 effectively enters the gap formed in the front-stage anodes 13 and the gap formed in the focus electrode 14 and acts on the electron beam.
According to this embodiment, due to the gap formed in the front-stage anode electrode 13 and the gap formed in the focus electrode 14, the magnetic field of the speed modulation coil 8 can easily enter the electron beam passing region. Further, an eddy current generated at the front-stage anode electrode 13 and the focus electrode 14 is reduced so that the sufficient speed modulation effect is obtained. Further, the influence derived from the bead glass and the connector can be suppressed so that a contrast of images is enhanced whereby an image display of high quality is obtained.
Further, in this embodiment, a focus electrode which has a short length in the tube axis direction is used as the second focus electrode 142 and the third focus electrode 143, the generation of an eddy current in the focus electrode 14 can be suppressed. insulation supporting body (bead glass) or a connector is increased so that a cross-sectional shape of the electron beams is deformed.
Since the interval between the divided halves of the electrode can not be increased, it is difficult to ensure the sufficient entrance of the speed modulation magnetic field into the electron beam passing region.
Further, when the total length of the cathode ray tube is short, the total length of the neck portion is short. Accordingly, it is difficult to arrange the speed modulation coil at a site close to a main lens and hence, a sufficient speed modulation effect can not be obtained.
Further, when the total length of the electron gun is short, the total length of the focus electrode is short. Accordingly, it is difficult to provide the sufficient gap for obtaining the speed modulation effect.
A cathode ray tube according to the present invention includes an evacuated envelope which is constituted of a panel portion on which a phosphor screen is formed, a neck portion which houses an electron gun and a funnel portion which connects the panel portion and the neck portion.
A deflection yoke, a correction magnetic device for correcting a track of electron beams and a speed modulation coil are exteriorly mounted on the evacuated envelope.
In the electron gun, a plurality of electrodes including a cathode, a control electrode, an acceleration electrode, a front-stage anode electrode, a focus electrode and an anode electrode are arranged at given intervals in the tube axis direction of the cathode ray tube. Each electrode is fixed by having an electrode support body which is mounted on a side wall thereof embedded in an insulation support body.
The front-stage anode electrode is divided into a plurality of portions (electrodes) in the tube axis direction of the cathode ray tube. A plurality of divided portions of the front-stage anode electrode are arranged at given intervals in the tube axis direction of the cathode ray tube and are electrically connected by connection lines.
The focus electrode is divided into a plurality of portions (electrodes) in the tube axis direction of the cathode ray tube. A plurality of divided portions of the focus electrode are arranged at given intervals in the tube axis direction of the cathode ray tube and are electrically connected by connection lines.
Due to such a constitution, an eddy current which is generated in the focus electrode due to a magnetic field generated by the speed modulation coil is reduced. Further, the magnetic field generated by the speed modulation coil can easily enter an electron beam passing region so that a sufficient speed modulation effect can be obtained. Accordingly, a contrast of displayed images is enhanced.
According to the present invention, it is possible to provide a cathode ray tube which exhibits a favorable contrast by enhancing the speed modulation effect.